I sure am getting alot of great casting tips from this group so far. I
wonder if any of you have ever had castings that turned out like this: my
casts are mostly ok but I get fair sized subsurface pits that appear when
polishing. Is this a gaseous problem or the metal freezing or investment
getting into the metal? Any ideas especially if you’ve had this problem
before would be helpful…Dave

I sure am getting alot of great casting tips from this group so far. I
wonder if any of you have ever had castings that turned out like this: my
casts are mostly ok but I get fair sized subsurface pits that appear when
polishing. Is this a gaseous problem or the metal freezing or investment
getting into the metal? Any ideas especially if you’ve had this problem
before would be helpful…Dave

Dave,
I am not sure about what is causing the problems you are having but
sounds sort of like incomplete burnout and water vapor possibly.Make
sure the flask is snowy white at the sprue opening before you cast and
when you look down into the sprue opening with the flask at full 1350
degree heat the reddish white glow should go all the way down thesprue.To me, they almost take on a transparency…Try soaking at the 1350
degree temperature for an extra hour or flip the flask up with the sprue
opening to the top for about a half hour at full heat(1350)Remember that
moisture rises as steam so by inverting the flask sprue opening up you
let the final moisture escape more readily. also try letting the 300-350
degree soak go on for an extra 30-60 minutes…If it is moisture
retention then this should solve the problem.Wonder what size flask you
are using? Also scape the top of the flask level before beginning the
burnout by using a sharp knife.A glaze forms on the top of the flask
during the first part of the drying(you can sometimes see it)which is
more dense than the rest of the investment and tends to prevent the
escape of moisture through the top of the flask.If you scrape the glaze
off it facilitates the evaporation of the remaining water upward and out
which lessens stress on the walls of the pattern cavity due to steam
being forced to escape though them instead of through the top of the
flask…Gavin.

Dave,
I am not sure about what is causing the problems you are having but
sounds sort of like incomplete burnout and water vapor possibly.Make
sure the flask is snowy white at the sprue opening before you cast and
when you look down into the sprue opening with the flask at full 1350
degree heat the reddish white glow should go all the way down thesprue.To me, they almost take on a transparency…Try soaking at the 1350
degree temperature for an extra hour or flip the flask up with the sprue
opening to the top for about a half hour at full heat(1350)Remember that
moisture rises as steam so by inverting the flask sprue opening up you
let the final moisture escape more readily. also try letting the 300-350
degree soak go on for an extra 30-60 minutes…If it is moisture
retention then this should solve the problem.Wonder what size flask you
are using? Also scape the top of the flask level before beginning the
burnout by using a sharp knife.A glaze forms on the top of the flask
during the first part of the drying(you can sometimes see it)which is
more dense than the rest of the investment and tends to prevent the
escape of moisture through the top of the flask.If you scrape the glaze
off it facilitates the evaporation of the remaining water upward and out
which lessens stress on the walls of the pattern cavity due to steam
being forced to escape though them instead of through the top of the
flask…Gavin.

Gavin: yes the cans are snow white and burnout looks complete. There IS
some strange contamination in the investment but figured out its like rust
from the flasks or actually when I vacuum the rubber bowl I get this
blackish looking stuff from the bowl itself I think but the bowl is brand
new, I still am leaning towards the beginning of the burnout cycle maybe
being too hot and too fast for my cans which are 2 1/2 inch little guys. I
think you have a better burnout oven than I do. I had four or five flasks
in last time and I noticed a difference between them I think as uneven
heating because of the exposed elements. I think you’ve given me enough
good ideas to try the next step and see if I can narrow down the
possibilities here, but I bet its investment getting into the mix…Dave

Dave,
I am not sure about what is causing the problems you are having but
sounds sort of like incomplete burnout and water vapor possibly.Make
sure the flask is snowy white at the sprue opening before you cast and
when you look down into the sprue opening with the flask at full 1350
degree heat the reddish white glow should go all the way down thesprue.To me, they almost take on a transparency…Try soaking at the 1350
degree temperature for an extra hour or flip the flask up with the sprue
opening to the top for about a half hour at full heat(1350)Remember that
moisture rises as s

Dave,
I am not sure about what is causing the problems you are having but
sounds sort of like incomplete burnout and water vapor possibly.Make
sure the flask is snowy white at the sprue opening before you cast and
when you look down into the sprue opening with the flask at full 1350
degree heat the reddish white glow should go all the way down thesprue.To me, they almost take on a transparency…Try soaking at the 1350
degree temperature for an extra hour or flip the flask up with the sprue
opening to the top for about a half hour at full heat(1350)Remember that
moisture rises as s

Xtalguy

Yer getting the metal too hot and forceing gas into the melt
*Joe,
Let me see if I remember my chemistry X quantity of sterling silver
absorbs only X quantity of gaseous Oxygen from melting point to boiling
and vaporization…Fine silver absorbs more oxygen per given weight than
does sterling.A small amount of phosphor-copper or sometimes called
black copper or phosphor-magnesium-copper or calcium boride will work
to remove any absorbed oxygen. …If you are using nitre in your flux
don’t do it…The only time to use the nitre is in a seperate melt and in
a seperate dish to remove copper but it is never a good idea to do so on
metal one intends to use in casting …Just get a good brand of casting
flux and a new crucible for silver and a seperate one for gold is not a
bad idea. Use only casting flux(they have dexodizers in them)in the new
crucible and oxygen absorption is not a problem…stir your metal with
a carbon rod when it looks like a bright shining pool of mercury and
don’t heat to the point where oxidation starts to swirl in patterns on
top of the melt just to the shiney point …Sterling starts smoking some
if you get it too hot so you can go by that in part as well…Gavin

Please explain ‘casting flux’ for an amature?? . . I add Borax w*
Jim,
I use Griffith brand casting flux but Rio Grande or several other
companies make a version of this as well.Yes it is different than plain
Borax and contains an anti-oxidizer and makes the crucible slippery in a
way that other flux doesn’t seem to for the spin casters…Gavin